Underwater acoustic impedance measuring apparatus

ABSTRACT

An automatic underwater acoustic impedance measuring apparatus is instrumented with transducers whose outputs are led to a computer for the automatic measurement of acoustic impedance. One hydrophone is positioned so that is senses the incident and reflected signals to compute reflection factor. A second hydrophone is positioned at the face of the sample material so that is senses the incident and reflected waves from which it is possible to compute the phase angle. Acoustic impedance is obtained from the phase angle and reflection factor, computed and printed out in real time and automatically swept through a frequency range of interest.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to an automatic underwater acoustic impedancemeasuring apparatus.

2. Description of the Prior Art

A pulse tube has traditionally been used as the apparatus for impedancemeasurement commencing with the efforts of Erwin Meyer and EugenSkudryzk, German professors, to develop underwater acoustic absorbersfor the German Navy during World War II. Physically, the pulse tube is athick walled steel structure filled with water and instrumented withacoustic transducers. A gated sine wave with about five cycles toapproximate a plane wave is the acoustic signal that is employed. Sometubes are equipped to provide temperature and hydrostatic pressurecontrol for studying samples subjected to varying environmental stimuli.

Typical dimensions of various pulse tubes are as follows:

    ______________________________________                                        Length (feet)    7     42       21   42                                       Inner Diameter (inches)                                                                        2     5.56     2.50 3.40                                     Wall Thickness (inches)                                                                        1     2.22     2.75 2.30                                     ______________________________________                                    

An acoustic signal is generated at the bottom of the tube to bereflected from the sample, mounted at the top, which is attached to ahigh impedance backing, or mounted against a layer of gas acting as alow impedance backing or mounted in the center of the tube so that thereis a water backing. A portion of the signal is reflected and its phaseis shifted by the sample. The percentage of energy reflected isexpressed as the reflection coefficient. Acoustic impedance can beobtained from a knowledge of the reflection coefficient and of the phaseshift.

The conventional method of measuring impedance in a water filledimpedance tube processes the reflected signal by nulling it with a knownsignal of equal amplitude and opposite phase. This is conventionallymanually performed by an operator at discrete frequencies and representsa long and tedious process subject to error and to operator fatigue.

One recent example, U.S. Pat. No. 4,305,295 illustrates a portableapparatus for measuring acoustic impedance in air at the surface ofcurved sound absorber wherein the apparatus consists of a circular,flexible disc held at a constant distance from a curved absorbingsurface by pins or flexible ribs. Sound from a loudspeaker is fed to thecenter of the discs and allowed to propagate radially in the spacebetween the disc and absorber. Radial arrays of microphones on the discsurface sense sound pressure amplitude and phase, from which impedanceis calculated. Such apparatus is used for determining the acousticproperties of absorbing linings as installed in ducts of jet engines. Itis also applicable for measuring the acoustic impedance of otherabsorptive surfaces, including for example, acoustic wall and ceilingpanels. Another example is illustrated in U.S. Pat. No. 4,289,143wherein there is described a method of and apparatus for audiometricallydetermining the acoustic impedance of a human ear in air.

The measuring of underwater acoustic impedance of various materials andacoustic structures is required in the use of such materials andstructures such as absorbers, decouplers, sonar domes, and transducers.The acoustic impedance of a material or of an acoustic structureprovides significant information regarding the operating parameters orattributes of the structure or of its components. The interpretation andexamination of an impedance locus reveals the magnitude of acousticabsorption, reflection, sonic velocity, elastic moduli, dissipation andresonance frequencies. These qualities are measured for purposes relatedto material development, for evaluation, for product development and forquality control. Some of the hydroacoustic applications derived fromacoustic impedance information are manifested as underwater absorbers,reflectors, transducers, sonar domes, and baffles. Dynamic moduli,useful in non-acoustic and in air acoustic applications, are alsodetermined from acoustic impedance information obtained with the use ofan underwater acoustic impedance measuring apparatus.

SUMMARY OF THE INVENTION

The present invention provides an automatic underwater acousticimpedance measuring apparatus comprising transducer means properlylocated on said apparatus, one transducer means positioned so that itsenses the incident signals and reflected signals to compute reflectionfactor, a second transducer means positioned at the face of samplematerial at the plane of said transducer means so that is senses theincident and reflected waves to compute the phase angle, said transducermeans properly interfaced with a computer means for automaticallycomputing and a printing means for printing out in real time theacoustic impedance.

OBJECTS OF THE INVENTION

An object of the invention is to provide an automatic underwateracoustic impedance measuring apparatus.

Another object of the invention is to provide an automatic underwateracoustic impedance measuring apparatus with enhanced performance.

Still another object of the invetion is to provide an automaticunderwater acoustic impedance measuring apparatus which automaticallycomputes and prints out a frequency range of interest.

Other objects and many of the attendant advantages and usage of theinvention will be readily observed and appreciated as the same becomesbetter understood by reference to the following detailed description.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a cross-sectional view of the impedance tube of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawing shows an embodiment of the automatic underwater acousticimpedance measuring apparatus of the invention. Three hydrophones,transducer means, are flush mounted in the wall of a steel tube that isfilled with water. A sample material is mounted so that its face is atthe plane of hydrophone H₁ when the backing is a high impedance such asa heavy mass or a low impedance such as a gas. The sample material maybe mounted so that its face is at the plane of hydrophone H₂ when thesample is water backed. If required, or desired for some technicalreason, a polymer film hydrophone or a flexible ceramic hydrophone ofvery small thickness can be attached to the face of the sample and itselectrical output can be led out through the backing located at the topof the tube, in place of hydrophone H₁ or hydrophone H₂.

The electrical signal produced by a gated sine wave at the hydrophonelocated at the sample face, H₁ or H₂, is the sum of the incident (Pi)and reflected signals (Pr). Incident pressure is "Pi". The reflectedsignal (Pr) is modified by the reflection factor "R" of the samplematerial and by its associated phase "θ". Reflection factor R=(Pr/Pi)².Through algebraic manipulation the magnitude of the hydrophone voltageis determined by the following formula:

    [Pi(1+2R Cos θ+R.sup.2)].sup.1/2

The reflection factor is obtained from the output of hydrophone H₃ byobtaining the ratio of the reflected to the incident signals.Consequently, it is possible to determine the phase angle from theoutput of hydrophone H₁, or H₂, and thus the acoustic impedance of thesample.

An integral part of the apparatus is a dedicated computer. First, afrequency range of interest is selected, e.g. 2k Hz to 9k Hz, with fineincrements such as 100 Hz and the dedicated computer is programmed tosweep the selected range automatically. Second, the reflection factor(R) is obtained from hydrophone H₃ and it is applied to the output of H₁or H₂ which computes the phase angle θ. Third, the values of "R" and "θ"are fed into a program for automatically calculating, printing andplotting acoustic impedance.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. An automated underwater acoustic impedancemeasuring apparatus comprising:(a) a first transducer means positionedwithin said apparatus for producing and transmitting a plane wave, (b) asecond transducer means positioned within said apparatus for sensingincident signals and reflected signals of a sample material placedwithin the apparatus for reflection factor computation, said samplematerial having a face perpendicular to a propagation direction of theplane wave, (c) a third transducer means positioned within saidapparatus at a plane containing the face of the sample material forsensing combined incident waves and reflected waves for phase anglecomputation, (d) computer means positioned and interfaced with saidsecond transducer means and said third transducer means forautomatically computing real time acoustic impedance in accordance withformula:

    R=(Pr/Pi).sup.2,

and

    [Pi(1+2R Cos θ+R.sup.2)].sup.1/2

wherein:R=reflection factor Pr=reflected sound pressure Pi=incidentsound pressure θ=phase angle of the acoustic impedance obtained from thereflection factor derived from the second transducer means and obtainedfrom the phase angle derived from the third transducer means, and (e)indicating means for indicating said acoustic impedance.